This invention relates to means for insulating commutator bars from their retaining rings in a commutator assembly of a dynamoelectric machine, and it relates more particularly to non-micaceous laminated commutator cones.
A typical commutator assembly such as a V-ring type commutator assembly comprises a plurality of electrically conductive commutator bars or segments which are mounted in a cylindrical array at one end of the rotatable shaft of a direct current (d-c) motor or generator for cooperation with relatively stationary carbon brushes. Adjacent bars are separated by insulating material. All of the bars are adapted to be held in their assembled relationship by a pair of metallic retaining rings having annular flanges adapted to extend into V-shaped notches formed in opposite ends of the commutator bars. A first one of the retaining rings, which are typically made of steel and in operation are at ground potential, is affixed to the machine's shaft, and the other ring is clamped to the first one by bolts or by other suitable means with the cummutator bars captured therebetween. The retaining rings are insulated from the commutator bars by suitable annular insulating members, generally known as commutator cones, which are disposed at opposite ends of the commutator assembly between the commutator bars and the retaining rings.
In operation the commutator cones must have good dielectric strength and good physical strength and stablility, since the commutator bars and the adjacent retaining ring between which each cone is located will subject it to a high steady-state electrical potential (e.g., 1,000 volts rms) and a high maximum mechanical compressive force (e.g., 6,000 psi). A typical commutator cone comprises an annular outer skirt integrally joined to a conical section so that the juncture thereof has a substantially V-shaped cross-section. The conical section is the part of the cone that is subjected to the most severe electrical and mechanical stresses.
Heretofore larger size cones have been manufactured to the desired V-ring contour by pressing and heating stacked layers or sheets of micaceous material in a suitably shaped mold. Prior to this process, individual pieces or flakes of mica are premolded into a plurality of flat segments of various predetermined shapes and sizes. To form the commutator cone, a first set of segments, along with a suitable binder (such as shellac or a synthetic resin varnish) which holds the segments together in a desired pattern, are arranged in an abutting manner to form a first generally ring-shaped lever, other sets of the segments are similarly arranged to form additional laminae on top of the first one (with the inter-segment joints or seams in each lamina being offset or staggered with respect to those in the preceding laminae), and then the stacked laminae are bonded together in the mold under pressure and heat. For a 12-inch diameter cone, these prior art premolding and final molding processes require about four to five hours.
Suitable large flake mica has become increasingly more difficult to obtain and more expensive. Mica flakes, as noted above, are premolded into segments of larger size, and consequently the mica segments have irregular surfaces. For this reason, and because of occassional resin "pockets" in the mica segments, the walls of the conical sections of micaceous cones are not as uniformly thick as is desired and some portions thereof may have light spots. The light spots may have insufficient dielectric strength to withstand the electrical stress to which the cones are subjected. Non-uniformity can result in out-of-round cones and "high spots" on the commutator (i.e., some of the commutator bars may protrude beyond a true cylindrical envelope), and this condition causes undesirably fast commutator brush wear. Furthermore, at those points between the retaining rings and the commutator bars where the cone wall is thickest, commonly referred to as the pressure points, the pressure against the cone is highest, thereby abrading the mica flakes and degrading their insulating property.
With micaceous cones and with micaceous insulation between adjacent commutator bars, the commutator has to be "seasoned" by repeated baking and tightening cycles, with the temperature and pressure being incrementally increased from cycle to cycle, whereby the manufacturing process consumes undesirable amounts of time and energy. In addition, due to differences in the coefficients of thermal expansion of copper, steel and the commutator cone, movements will occur when the temperature rises and falls during machine operation. Such relative movements can lead to the grinding destruction of the brittle mica flakes in the prior art cones.
In order to improve the uniformity of the physical and electrical properties of a micaceous commutator cone and to avoid pressure points and so-called resin pockets, P. R. Gilbert has suggested, in his U.S. Pat. No. 3,500,094, that a lamina of uncalendered polyamide fiber paper be disposed between exterior layers of mica. Prior to final curing, the interior lamina is more compressible than mica and therefore tends to cushion and compensate for thickness variations in the micaceous layers.
In another prior art commutator cone of which I am aware, the segments or sheets of mica are replaced altogether with paper-like sheets of small aramide fibers, bonded together with polyimide varnish. The resulting cone is referred to as a Nomex V-ring. The raw material can be purchased in uniformly thick sheets that can be cut into segments having the various predetermind shapes that are desired for molding purposes, thereby eliminating the cost of premolding mica flakes into larger segments and avoiding the irregular surfaces and the light spots of such premolded micaceous segments. However, the wall of a Nomex V-ring has seams due to the piecing together of a plurality of individual segments of aramid sheets, and it is nearly as thick as the wall of a micaceous cone. Furthermore, the mechanical and electric-insulating properties of a Nomex V-ring tend to degrade when the V-ring has aged at elevated temperatures.
In his U.S. Pat. No. 2,528,235, J. A. Loritsch has disclosed a non-micaceous commutator cone comprising laminated sheets of glass-fiber cloth bonded together with a polyvinyl acetal resin-modified copolymer of a polymerizable unsaturated alkyd resin and a polyallyl ester. The dielectric strength of this composite material at high temperatures is undesirably low.
An insulating material that is advantageous in a high-temperature environment is known generically as polyimide film, and it is manufactured and sold by the DuPont Company under the trademark "Kapton." An FEP-fluorocarbon resin coated form of such film is made by the DuPont Company. This material will remain physically and electrically stable at higher temperatures and has a higher dielectric strength than Nomex. However, it is not readily formable into a body having the mechanical strength, stiffness, and shape of a commutator cone.